Bird calls drowned out by city noise
BIRDS IN CITIES ARE struggling to communicate, drowned out by the rumbling of traffic. For male songbirds, singing loudly and clearly attracts a mate – so the consequences can be dire. “Noise is a big barrier,” says Dominique Potvin, an expert on birdsong at the University of Melbourne. “If you’re not being heard, you can’t get mates and you can’t reproduce.”
But some species are coping. New research shows urban songbirds are singing louder and at a higher pitch than country-dwelling relatives. They can make these adjustments because their songs are learnt and can change in response to the environment. Bird calls, on the other hand – used to keep in contact and warn off predators – are less flexible and more genetically determined.
In 2011 Dominique and her colleagues published the first study that looked at the way songs and calls differ between urban and rural birds of a single species: the silvereye. Changes in song would reflect behavioural flexibility, the researchers reasoned, whereas changes in the innate call would suggest recent, rapid evolution.
Bird calls changed by city sounds
They recorded calls and songs of 14 silvereye populations across 1 million sq.km of south-eastern Australia, and found that city silvereyes sang and called at a higher pitch. Songs were on average 195 Hertz (Hz) higher and calls 90Hz higher in cities. Urban noise is low-frequency (1000–4000Hz), which is bad news for silvereyes, whose lowest notes sit in the 2000–3000Hz range. By raising the frequency of songs and calls, they reduce the masking effect of city noise.
“Urban habitats are directly influencing the evolution of vocalisations,” says Dominique. Over time, vocalisations between urban and rural birds could diverge to the point where they can no longer communicate, she says, “potentially spurring genetic isolation and divergence into different species”.
Another distinction is that city silvereyes sing slower and, possibly, louder. The hard surfaces of buildings and roads echo birdsong, so the birds pause between syllables to let notes ring out clearly.
Recent studies have found higher vocalisations in urban rainbow lorikeets, eastern rosellas and the grey shrike-thrush. “If you listen to how the grey shrike-thrush sings at quiet and noisy sites, it’s a clearly audible difference,” says Dr Kirsten Parris, a University of Melbourne ecologist. Kirsten’s work has shown that naturally high-pitched birds, such as the grey fantail, have little need to adapt because their calls compete less with background noise.
But for threatened species struggling to stake a claim, mitigating traffic noise could be a viable conservation measure. Dominique says town planners should consider how noise affects animals that rely on acoustics, with measures such as smoother roads. “We need to rethink the idea that all we need is good habitat and good tree cover,” she says. “We have to take noise seriously as well.”
Source: Australian Geographic Mar – Apr 2012
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